Determining Phytophthora Resistance in Elite Paradox Clones and Specific Etiology of a Paradox Crown and Root Rot

The objectives of this research are: (1) to evaluate elite Paradox hybrid clones for resistance to Phytophthora citricola, a virulent and prevalent crown and root pathogen affecting California walnuts and (2) to determine specific etiology of a root and crown rot problem in upper Sacramento Valley orchards on Paradox walnut rootstock. Two screens for resistance to P. citricola were completed in 2004, each including 10 clones of diverse walnut hybrids from black (maternal, M) and English (paternal) parents. The clones were selected from seed families for one or more traits, including putative tolerance to P. citricola, Cherry Leaf Roll Virus, or Pratylenchus vulnus; or unique parentage. The selections were propagated as micro shoots, rooted, acclimatized and grown in a greenhouse in 4 x 20 cm cones (3 months), subjected to dormant chilling (6 °C, 3 to 5 months), and allowed to resume growth in a greenhouse (2 to 3 months) before inoculation with P. citricola. The plants were transferred from the cones into 2liter pots filled with non-infested soil (6 plants per clone) or soil artificially infested with P. citricola (12 per clone). Starting 2 weeks after transplanting, all plants received biweekly 48-h periods of soil flooding. Three months after transplanting, each plant was washed free from soil and rated for severity of crown and root rot. Severity of crown rot was affected by interactions of inoculum × clone (P<0.0001) and experiment × inoculation × clone (P=0.05). Across experiments, non-inoculated controls developed little disease (means 0 to 5% crown length rotted), but inoculated clones developed moderate to severe crown rot. Highly susceptible clones (means 52 to 90% of crown length rotted) included AX-1 (M=J. californica); GZ-1 and PX-1 (M=J. hindsii); and WIP3 (M=J. hindsii x regia). Moderately tolerant clones (20 to 43% crown length rotted) included AZ-2, AZ-3, and NZ-1 (M= [J. major x hindsii] x nigra); JX-2 and VX211 (M=J. hindsii); and RX-1 (M=J. microcarpa). The results, obtained under conditions highly conducive to the disease, suggest that the moderately tolerant clones have potential as improved Paradox rootstocks in orchards infested with P. citricola. Towards completion of the second objective, multiple samples were collected from six Sacramento Valley walnut orchards in areas where English walnut trees grafted on Paradox hybrid rootstock were declining with symptoms of crown and root rot. Although overall incidence of detection of a Phytophthora sp. was low from these samples, one or more Phytophthora sp. was detected from five out of the six orchards involved. The isolates were tentatively identified as P. parasitica and an unknown Phytophthora sp. Representatives of the isolates will be subjected to further characterization and tested for pathogenicity on Paradox hybrid and Northern California black walnut seedlings. Although complicated by commercial applications of phosphonates, isolations for Phytophthora will continue in the affected walnut orchards. INTRODUCTION Crown and root rots caused by species of Phytophthora are among the most serious diseases of walnut worldwide. In California, more than 10 species of Phytophthora have been implicated in the diseases, but P. cinnamomi and P. citricola were determined to be the most virulent (Mircetich et al., 1998). There has been continued interest in comprehensive evaluation of Paradox hybrids for resistance to Phytophthora spp. and other desirable traits. Paradox has proven more resistant than Northern California black or English seedling rootstocks to most Phytophthora spp. Although Chinese wingnut is the only walnut family relative known to tolerate P. cinnamomi, it is not graft compatible with all English walnut cultivars and has other potential limitations (i.e., suckering, unknown yield efficiency). Paradox hybrids available from commercial nurseries are diverse, involving crosses between J. regia and one or more species of black walnut (Potter et al., 2002), and results of greenhouse experiments suggested that the diversity among Paradox hybrids may include important variation in resistance to P. citricola (G.T. Browne, unpublished results). Development and application of propagation and acclimatization technology by Wes Hackett and the Walnut Improvement Program (WIP) provided rooted hybrid clones from previous selections made by us, the WIP, and M.V. McKenry. Here we report on: (1) results of replicated evaluations of resistance to P. citricola in Paradox hybrid clones (previously unreported results from 2002-03 and results from 2004 are summarized), and (2) results of field surveys to determine specific etiology of a crown and root rot on Paradox hybrid rootstock in the Sacramento Valley.